Well stimulation fluid and well stimulation fluid recycling process

ABSTRACT

A glycol-based well stimulation fluid and a process for recycling a glycol-based well stimulation fluid, comprising the steps of filtration and heating.

FIELD OF THE INVENTION

[0001] This invention relates to the field of oil and gas production,and more specifically to the improvement of the properties and/orperformance of well stimulation fluids and the recycling and re-use ofwell stimulation fluids.

BACKGROUND OF THE INVENTION

[0002] Well stimulation is the treatment of a formation in order torestore or enhance the production of oil or gas. One commonly usedmethod for enhancing productivity is to subject the formation tohydraulic fracturing. Typically, a fracturing fluid or well stimulationfluid is injected into the well at a rate sufficient to open a fracturein the exposed formation. Continued pumping of fluid into the well at ahigh rate extends the fracture.

[0003] The fluid may contain a propping agent, so that fluid injectionleads to the build up of a bed of propping agent particles between thefracture walls. These particles prevent complete closure of the fractureas the fluid subsequently leaks off into the adjacent formations or isrecovered back to the surface and results in a permeable channelextending from the well bore into the formation.

[0004] The fluids used in hydraulic fracturing operations must havefluid loss values sufficiently low to permit build up and maintenance ofthe required pressures at reasonable injection rates. This normallyrequires that such fluids either have adequate viscosities or otherfluid loss control properties which will reduce leak-off from thefracture into the pores of the formation.

[0005] Oil and gas well stimulation fluids are used for a variety ofreasons to increase the mobility of the reservoir fluid toward thewellbore. Acids are used for their reactivity with the hydrocarbonbearing formation to increase permeability by removing formationmaterial. Solvents and other reactive agents are used to removeprecipitates due to the production of reservoir fluids (asphaltines,waxes, scale). Methanol and other surface reactive agents (surfactants)are used to reduce problems associated with water blockage within theformation. Fracture stimulation employs a variety of fluids and gasesindividually or in combination to propagate an initiated fracture and asa medium to transport the proppants required to provide the permeabilitycontrast to increase production rates from the reservoir to thewellbore. The ideal stimulation fluid would perform the functionsrequired of it and then be recoverable with no residuals left in thereservoir.

[0006] The most common fracture fluid is water, due to its relativelylow cost, safety and availability. Water however can be trapped withinthe reservoir rock to reduce permeability, thereby not providing astimulation to the full potential. Surfactants can be added to the waterto reduce its surface tension, but increase cost and safety concerns(flammability). Other chemicals are also used to enhance properties suchas viscosity, but can also leave residuals. Water adds costs when usedin cold weather as it requires heating to the 20 C range for optimumblending with chemical additives.

[0007] Hydrocarbons, crudes, reformates and diesels, are also used asfracture fluids but at higher cost of fluids and safety. Again chemicalscan be added to enhance properties but have been seen to cause problemsin downstream facilities such as refineries.

[0008] Gases and liquid gases such as nitrogen, liquid carbon dioxide,and propane have been used as energizers, fillers, and principle fluidsfor stimulations. Nitrogen is basically inert, and its properties as agas provide a method to enhance stimulation fluid recovery due to itsrelatively low specific gravity. Liquid carbon dioxide works in the samemanner, but is pumped as a liquid and is recovered as a gas. Liquidcarbon dioxide also acts as a solvent and surfactant. Liquid carbondioxide has been used as a principle fluid and in combination withnitrogen as a filler, but requires specialty equipment to perform theoperation. Liquid carbon monoxide and nitrogen are not flammable andtherefore reduce the safety concerns but the gases are not recyclable.Propane has also been used for its solvent properties and as a principlefluid, but is a high safety risk.

[0009] Methanol is used as an additive, filler and as a principle fluid.Methanol works as a surfactant, reducing surface tension of water, andis fairly cost effective. It can be used to reduce the freezingtemperature of a water solution and as a filler to reduce the amount ofwater used in an operational treatment. Methanol however increasessafety concerns proportional to the concentrations used. Chemicals havebeen used to increase its viscosity, but methanol vaporizes at formationtemperatures making the recovery of the chemicals very difficult.

[0010] Liquid CO₂ is used as a non damaging fully recoverable fracturefluid. Gaseous nitrogen has been added as a filler to help reduce thecosts of the treatment. And a treatment was patented that used achemical to foam the liquid CO₂— nitrogen gas mixture. This processrequire specialty equipment and the fracture fluids are not recyclable.

[0011] As discussed above, one problem encountered in fracturingoperations is the difficulty of total recovery of the well stimulationfluid. Fluids left in the reservoir rock as immobile residual fluidsimpede the flow of reservoir gas or fluids to the extent that thebenefit of fracturing is decreased or even eliminated. The removal ofthe well stimulation fluid may require the expenditure of a large amountof energy and time, consequently the reduction or elimination of theproblem of fluid recovery and residue removal is highly desirable.

[0012] One way of addressing this problem is to reduce the frictionbetween the fluids and solids introduced into the well during drillingor subsequent treatment and the formation fluids and solids. This hasentailed the use of surfactants to reduce surface tension between, forexample, liquid/liquid and liquid/solid interfaces.

[0013] Another problem is that even after the well stimulation fluid isrecovered, it may not be easily and economically recyclable. Theinability to reuse stimulation fluid increases treatment costs bynecessitating the disposal of used fluid and the purchase of new fluid.

[0014] Well stimulation fluids are commonly water-based or oil-based,and often contain a range of additives such as surfactants, fillers, andproppants. The disadvantage of water as aforesaid is that it must beheated to 20° C., resulting in higher costs when drilling takes placeunder cold conditions. In addition, it is not always easily recovered.Also, as is well known, water is simply incompatible with certainformations and can cause swelling, particularly in the clays. Oilcreates safety concerns, due to its flammability. Neither water-basedfluids nor oil-based fluids are recyclable at a reasonable cost.

[0015] What is needed is a well stimulation fluid that is suitable foruse under typical temperature ranges, has low flammability, is easilyrecovered from the formation, and is easily recycled for reuse at areasonable cost.

[0016] A method for recycling the well stimulation fluid is also needed.

SUMMARY OF THE INVENTION

[0017] The present invention relates to a well stimulation fluidcomprising glycol. The term “glycol” is used herein to refer to adihydric alcohol in which two hydroxyl groups are bonded to differentcarbon atoms. The general formula takes the form:

(CH₂)_(n)(OH)₂

[0018] Any glycol may be used in the fluids of the present invention.Generally, the larger the glycol molecule, the higher its viscosity.Larger glycol molecules also tend to be more costly.

[0019] The simplest glycol is ethylene glycol:

CH₂OH CH₂OH

[0020] Ethylene glycol is commonly used as antifreeze in automobilesbecause of its relatively high boiling point (197 C), low freezingpoint, miscibility with water and low corrosive activity.

[0021] Environmental aspects of ethylene glycol show well relative toother products used in oil and gas production. Ethylene glycol reactswith hydroxyl radicals with a half life of the reaction estimated at 0.3to 3.5 days.

[0022] Historically glycol has been used in gas production as a productto help eliminate water in gas pipelines. Glycol is injected prior to adehydration unit, where the glycol water solution is heated to removethe water and then re-injected for the same purpose. In some cases,hydrocarbon condensate is also a by product and is used as a fuel forheating the solution. In recent years glycol has been used as a heattransfer agent replacing the conventional use of boilers and steam forwinter heating operations. The glycol system is more efficient as it isa closed loop system, reusing the fluid.

[0023] Glycol has also been used as an additive or carrying agent foradditives used in stimulation fluids. The surfactant nature of theproduct has been used for foaming and defoaming aerated fluids and as anon aqueous carrier for solid particles such as cement.

[0024] The high boiling point of glycols means that fewer vapours aregenerated during well stimulation. Glycol reduces the freezing point ofaqueous solutions; this is particularly important when gas/oilproduction is taking place at sub-zero temperatures. It also reduces thesurface tension of aqueous solutions, which improves fluid recovery. Itis soluble in many substances commonly used in stimulation fluids, suchas water, methanol and toluene. Glycol has a high flash point, meaningthat it is non-flammable and safe to use. Its viscosity is inverselyrelated to temperature, so it is not necessary to heat the fluid inorder to increase viscosity.

[0025] Glycol may be used on its own as a stimulation fluid, or incombination with additives. For example, because of its high density,energizers such as N₂ and CO₂ may be used to improve recovery. Fillersand solvents such as water, methanol and toluene may be added to reducethe amount of glycol needed, thus reducing costs. Surfactants such asSurfynol™ 124, available from Air Products, may be added to reducesurface tension. Various chemicals may be added to enhancecharacteristics such as viscosity or foaming, or to reduce side effectssuch as damage to the formation. None of these additives would affectthe recoverability and recyclability of glycol.

[0026] Although the glycol-based fluids of the present invention are tobe used mainly for well stimulation, they may also be used for anyapplication in which it is advantageous to recover and recycle thefluid. For example, glycol-based fluids may be used as kill fluids,where it is desirable to stop the flow of reservoir fluids. Glycol-basedfluids may be used as workover fluids, where the well is being repairedor restored. In addition, the fluids of the present invention may beused to transport solids, such as mineral processing wastes, backfillfor open pits, clay for lining storage ponds, substances for sealingcoal fires, and materials for filling mined cavities to prevent surfacesubsidence.

[0027] The present invention also relates to a process for recycling awell stimulation fluid comprising glycol. Upon recovery from the well,the well stimulation fluid is channelled into a separation vessel wheresolids fall to the bottom of the vessel, liquids are drained off, andgases are removed through the top of the vessel. The liquids are thenfiltered to further remove any solids. The liquids are heated in aregeneration vessel to a point where water and hydrocarbons arevaporized, yet glycol remains in a liquid state. The gaseous materialsare removed from the top of the vessel. This process may be repeateduntil the desired purity of glycol is achieved. It can then be removedto storage, awaiting reuse.

[0028] Glycols are relatively more expensive than fluids such as water,methanol, liquid CO₂, and hydrocarbon fluids. Glycol is approximatelydouble the cost of methanol and liquid CO₂. The cost benefit toutilizing glycol is the relative ease in recapturing and recycling itafter the stimulation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a schematical view of the equipment to be used forblending and pumping the well stimulation fluid.

[0030]FIG. 2 is a cross section of a well bore illustrating the flow ofwell stimulation fluid into the formation.

[0031]FIG. 3 is a cross section of a well bore illustrating the flow ofwell stimulation fluid and gas or oil out of the formation.

[0032]FIG. 4 is a cross section of a separation vessel.

[0033]FIG. 5 is a cross section of a regeneration vessel.

DETAILED DESCRIPTION OF THE DRAWINGS

[0034] The present invention proposes the use of glycol as a stimulationfluid additive or as a principle fluid utilizing conventional equipment.Further, the processes would utilize recapture and recycling operationsto reduce the overall cost of the operation and reduce the volume offluids requiring additional disposal costs.

[0035] Ideally a glycol would be used as the sole stimulation fluidwithout the addition of chemicals. This type of stimulation would becomparable to a liquid CO₂ treatment, but not require the specializedequipment needed to add proppants to the fluid. Viscosity limitations ofliquid CO₂ (about 0.1 cP) are not a problem, and glycol viscosityincreases with lowered temperatures (no heating required). Posttreatment, the glycol is easily and safely collected where it can berecycled by heating to remove any absorbed water and other impurities.

[0036] Due to the density of glycols, it is envisioned that energizerssuch as nitrogen and liquid CO₂ will sometimes be used to help in therecovery of the treatment fluid. The use of liquid CO₂ could enhance theviscosity properties of the glycols as liquid CO₂ is pumped at −20 C.Again the recovered fluid would be easily captured and heated forrecycling.

[0037] Spacers (fillers) and/or solvents and/or surfactants, water,methanol, toluene or other commonly used stimulation fluids may be usedto reduce costs or enhance the required characteristics of the glycol.Still the glycols would be recoverable and recyclable.

[0038] Glycol could also be used as a spacer (filler) to enhance theproperties of other commonly used stimulation fluids.

[0039] It is also envisioned that chemical additives can be used toenhance the properties of glycols (for example: viscosity) or reduce therisks of potential damaging side effects caused by non pure recycledproducts (examples biocides, clay swelling . . . ). Chemical additivescould be created to optimize the foaming characteristics when used withgases or liquid gases.

[0040]FIG. 1 illustrates that glycol is stored in tanks 1 prior to use.When the glycol is needed, it is directed through a blender unit 2 whereit is blended to ensure that the glycol and any additives and proppantsare throughly mixed. A fluid pumper unit 3 then pumps the glycol fluidthrough a high pressure line 4 to the well head and into the formationbeing treated at a pressure sufficient to cause fracturing as is wellknown in the art. Energizers such as liquid CO₂ and N₂ may also bepumped from a gas source pumping unit 6 into high pressure line 4 alongwith the glycol fluid. Pumping the CO₂ or N₂ from a separate sourcepermits independent control of the timing and rates at which thesefluids are introduced into the glycol stream.

[0041]FIG. 2 illustrates the flow of the well stimulation fluid 8 downthrough the well bore 7 into formation 9 to instigate and propagatefractures into the formation.

[0042]FIG. 3 illustrates the flow back of well stimulation fluid, alongwith water and gas or oil from formation 9, back up through well bore 7to the surface.

[0043]FIG. 4 illustrates that as the well stimulation and othermaterials flow out of well head 5, they are diverted into a separationvessel 10. Gaseous materials are removed through the top 11 of thevessel and may be either flared off or channelled to a pipeline forfurther processing. The presence of glycol in any recovered gas is not aproblem as the industry typically uses glycol in any event to removewater from flowing gas. Solid materials fall to the bottom 12 of thevessel. The remaining liquids may be drained out for furtherpurification as shown in FIG. 5.

[0044]FIG. 5 illustrates that the after leaving separation vessel 10,the liquid may be passed through a filter 13 to further remove anyremaining entrained solids. The filtered liquid is then collected in aregeneration vessel 14. The liquid is circulated between regenerationvessel 14 and a combined heater and circulation pump 15. As the liquidis heated, water and hydrocarbon materials having a boiling point lowerthan that of glycol will vaporize. The vaporized materials are evacuatedthrough a valve 16 at the top of vessel 14. Circulation of the liquidcontinues until the desired purity of glycol is achieved, at whichpoint, the purified glycol 18 is channelled off for storage and reuse.

[0045] Although its contemplated that ethylene glycol will be used mostcommonly in the context of the present invention, other glycols can alsobe used including, but not limited to diethylene, triethylene,tetraethylene, propylene, dipropylene and tripropylene glycol.

[0046] The above-described embodiments of the present invention aremeant to be illustrative of preferred embodiments of the presentinvention and are not intended to limit the scope of the presentinvention. Various modifications, which would be readily apparent to oneskilled in the art, are intended to be within the scope of the presentinvention. The only limitations to the scope of the present inventionare set out in the following appended claims.

I claim:
 1. A well stimulation fluid comprising a glycol, said a glycolrepresenting between 51 and 100% of said fluid, by weight.
 2. A wellstimulation fluid as claimed in claim 1, in the form of a foam includinga liquefied gas selected from the group consisting of N₂, CO₂, and air.3. A well stimulation fluid as claimed in claim 1, further comprising asolvent selected from the group consisting of water, methanol, andtoluene.
 4. A well stimulation as claimed in claim 1, wherein saidglycol is ethylene glycol.
 5. A kill fluid comprising glycol.
 6. Aworkover fluid comprising glycol.
 7. A fluid for transporting solids,comprising glycol.
 8. A method for recycling a well stimulation fluidcomprising a glycol, said method comprising the steps of: a) filteringsaid fluid; and b) heating said fluid to vaporize materials having alower boiling point than said fluid.
 9. A method as claimed in claim 8,wherein said glycol is ethylene glycol.